1. Field of the Invention
The present invention relates to an electron beam apparatus and an electron beam adjusting method for adjusting an electron beam into a desired shape to be irradiated on a target.
2. Description of the Related Art
An electron beam apparatus using an electron beam is widely applied to manufacturing apparatuses such as an electron beam drawing apparatus in the semiconductor device manufacture, and a measuring apparatus such as an electron microscope, since a beam spot diameter can be adjusted to be extremely small. Also, development is now under progress to an application of the electron beam apparatus to a mastering apparatus for a large capacity disc such as a digital versatile disc (DVD), a hard disk drive for magnetic recording, and the like. The electron beam apparatuses are widely used in measuring apparatuses such as a scanning electron microscope (SEM) and evaluation apparatuses.
For example, in a mastering apparatus for manufacturing a master disc of the DVD disc and the like, the accuracy of widths of pits and grooves to be recorded (i.e., cut) is critical, so that the diameter of a recording beam must be precisely controlled for achieving a high accuracy. While an electron beam cylinder or column of the electron beam apparatus is designed to provide a predetermined beam diameter, a precise adjustment is required for an optical axis (electron beam axis) for achieving the design performance. For this reason, the beam diameter is typically measured before starting the recording and adjusted to fall within a predetermined range of values.
As described above, the beam diameter, aberration and focus must be accurately evaluated and adjusted for achieving a high accuracy in the electron beam apparatus. The beam diameter can be measured, for example, by observing a sample having a miniature structure using a SEM function of the electron beam apparatus, and estimating the beam diameter from the resolution of a SEM image of the sample. However, the measurement is problematic in reproducibility due to arbitrariness in adjustments of contrast and brightness of a SEM image display system. Therefore, though used in research applications, an application to manufacturing apparatuses and the like still includes problems.
The beam diameter may be measured by another method which uses an edge signal. More specifically, a beam is one-dimensionally scanned on a sample which has a vertical and smooth edge, and the beam diameter can be evaluated using a change in the intensity of a detection signal generated near the edge. While several methods are available for detecting means, the most successful one is a knife-edge method which uses a sample made of silicon or the like and having a hole extending therethrough or a cross wire and measures a change in the intensity of a current which reaches a lower part thereof. Specifically, a beam irradiated vertically to a knife edge (for example, a cross wire such as a tungsten wire) is scanned to measure the waveform of a current which reaches a lower part of the knife edge. The beam diameter is calculated from a rising time of a step-shaped signal waveform, and a scanning speed. Actually, however, this method suffers from an insufficient S/N (signal/noise) ratio of the detection signal which largely fluctuates due to noise. For a more accurate measurement, the waveform of the current must be measured a plurality of times for averaging, however, a long time is required for achieving an accurate measurement.
The foregoing method uses a sample having edges orthogonal to each other (along an x-axis and a y-axis, respectively), scans a beam vertically to the edge in the y-direction to evaluate the beam diameter in the x-direction, and similarly scans the beam vertically to the edge in the x-direction to evaluate the beam diameter in the y-direction.
However, in the evaluation of the beam diameter in the orthogonal directions, aberration symmetric about the axis cannot be separated from aberration asymmetric about the axis. In addition, defocusing (i.e., out-of-focus) cannot be separated from aberration. For example, as shown in FIG. 1, the beam diameter (profile) is extended so that a measured value appears large due to both edge signals in the directions orthogonal to each other (x- and y-directions) when the beam is out of focus (FIG. 1B) as compared with when the beam is focused without astigmatism (FIG. 1A). Likewise, when there is astigmatism asymmetric about the axis (for example, the beam extends at an angle of 45xc2x0 from the x-axis or y-axis, as shown in FIG. 1C), the beam diameters in the x- and y-directions are likewise observed to be large, so that a large measured value is generated from the edge signal. For this reason, it is not possible to determine from the measured values of the beam diameters in the x- and y-directions whether to make adjustment for either of defocusing or astigmatism. Also, as described above, for finding a precise beam diameter, the measurements must be repeated a proper number of times for averaging, giving rise to a problem that a long time is required therefor.
The present invention has been made in view of the foregoing problems, and it is an object of the present invention to provide an electron beam apparatus and an electron beam adjusting method which are capable of accurately and rapidly adjusting the focus and aberration of an electron beam.
To achieve the object, according to one aspect of the present invention, there is provided an electron beam apparatus for irradiating a target with an electron beam, comprising a reference sample including at least one reference pattern which has a plurality of lattice structures arranged along the circumference of a circle in a evaluation surface of the reference sample; and an adjustment section for adjusting the electron beam by irradiating the evaluation surface with the electron beam on the basis of electrons generated from the reference sample.
To achieve the object, according to another aspect of the present invention, there is provided a method of adjusting an electron beam for irradiation of a target, comprising the steps of providing a reference sample including at least one reference pattern which has a plurality of lattice structures arranged along the circumference of a circle in a evaluation surface of the reference sample; scanning the electron beam on the plurality of lattice structures within the same reference pattern; converting a change in electrons caused by the scanning operation into an electric signal; and comparing a plurality of waveform blocks corresponding to the respective lattice structures within the electric signal to adjust the electron beam such that the waveform blocks become uniform.